Lens.



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R. D. GRAY.

LENS.

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lllill'l l WTNESSES %u/ UNITED STATES PATENT OFFICE;

ROBERT D. GRA'Y, oF NEW YORK, N. if.

LENS.

srmncsnos forming para of retten Patent No. senseo, ama september 1 1, ieoo. Application iisd July' 16,1899. Serial le. 728,0. (le medal.)

To all whom it rrtay concern,.-

Beit known that I, ROBERT D. GRAY, ofthe city of New York, borough of Manhattan, in the county and State of New York, have inl5 vented a new and useful Improvement in Lenses, of which the following isa full, clear, and exact description.

The invention relates to optical objectives for use in photographic apparatus, telescopes,

1o and other opticalinstruments; and the object of the invention is to provide certain new and useful improvements in lenses, whereby the eective aperture of the objective is greatly increased and spherical and oblique aberration and astigmatism are reduced to a minimum.

The invention consists of novel features and parts and combinations of the same, as

,` will be fully described hereinafter and then so pointed out i'n the claims. 4

A practicalembodimentof the invention is represented in the accompanying drawings, forming a part of this speciiicatiom'in which similar characters of reference indicate cor- :5 responding parts in both views.

Figure 1 is a sectional side elevation of an objective for a photographic camera, show-v ing my improvement. Fig. 2 is a like view of a modified form of the same.

3o Each element of an optical objective for photographic or other apparatus as heretofore constructed consisted, essentially, of a collecting, condensing, or positive lens of crown glass and a dispensing or negative lens of iiint glass, the negative lens being of a higher refractive index than the positivelens. Objectives constructed in this manner are seriously defective, and the spherical and oblique aberration increases with an attempted 4o increase of eective aperture, and in order to overcome this defect i construct each eleinent with a positive lens having ahigher refractive index than that'of the negative lens. Glasses of high refractive index require longer radii-of curvature for a given focal length than those of less refractive index. Consequently there ls less spherical aberration in auch a lens, and glasses of low refractive lndex requlre shorter radii of curvature for a 5e ven focal length with an increase of sphercal aberration.. It is now' evident that if a glass of the lowest refractive index be used for the negative lens and a glass of higher index be used for the positive lens then the positive spherical aberration of the positive lens can be corrected for a relatively-greater diameter without destroying the positive nu ture of the combination than can be done by the reverse proceeding as heretofore practiced. Another advantage gained by con# 6o strucliug the objective as described is a like reduction of oblique sphericalaberration and a relative lengthening of the focus 0f those' rays which pass through the latitudinal section of the lenses, thereby reducing astigmatiem to a minimum. v

As shown in Fig. 1, the two elements A A' are each provided with a positive lens B, of crown glass, and a negative lens C, nl' flint glass, the lenses being spaced lapart to form 1c an air-space D between the same,as is plainly indicated in said Fig. 1. The positive lens B has a higher refractive index than the negative lens C, for the purpose previously mentioned. v A 15 The objective shown in Fig. 2 is provided with two elements A A, each having a positive leus B', a negative lens C', and a.third lens E, preferably in the form of a meniscus lens and of, a low refractive index similar to 8c that of the lens C', but with a dispersion to allow of making secondary corrections, and consequently rendering the element more achromatic. An air-space D' is left between the lenses B' and C in each element A' A'. 8

It is understood that lf the lenses B and C were placed in contact with each other without an inrerveningair-space then pronounced astigmatism would be the consequence, and hence separating the lenses is a very ilupcr- 9o tant feature of my invention. The amount ci this separation of the lenses varies with the thickness of either of the lenses or all of the lenses in the combination and with thealnonpt of separation between the elements. Now by having the lenses ln each element separated allows more freedom in the selection 0f the inner curves, as it is evident that if there is no space the adjacent or contacting faces of the lenses must be alike in cnrvature.- New es by the sance between the lenses the curvature of thea jacent facescan be varied tolengthen s' senso the focus of the oblique rays of light independently of the Axial focus to minimize or cure astigmatism.

In order to produce an achromatic objective 5 with the glasses most suitable for eliminating the aberrations referred to, it is necessary to construct the lenses with relatively-deeper xo and described i n Fig. 2. By having the third longer fora given focus in the compound lens t5A than would be required in a simple positive lens. 'lhe object of lengthening the radius of this surface is the further correction of sstigmatism. I prefer that the lens E shall :o lens B, which permits the lens C to be of lower As shown in Fig. 2, the lenses C' snd B' sre of similar form to the lenses snd C, the lens" 35 being of meniscus form, ss previously mentioned.

It is understood that for photographic objectives two elements are usually employed, but for telescope and other objectives only 4o one is necesss Having thus fullv described myinvention,

` .I claim as new sud desire to secure by Letters Putent-v l 1. In s n element for an optical objective, 45

positive lens having s higher refrsctive index than the ssid negative lens.

2. In an element for sn optics! objective, 5e

positive lens consisting of two sections, eneof l the sections ol'- said positive lens being of higher refractive index than its other section 55 and than the negative lens. n 3. An element foran optical objective, oomprising a plano-convex positive lens end s spsced spart toleavesplsnoeonvex slr-space 6e between the lenses, the positive lens having s higher refractive index than the negative lens.

'ROBERT n.' een. Witnesses:

Tano. G. Evmzsnp Bonson Husum. 

